Stereophonic sound reproducing system

ABSTRACT

A stereophonic sound reproducing system having left and right audio transducer means arranged relatively close to each other, and a combiner means connected to a stereophonic sound signal source for generating left and right signals R and L and combining the left and right signals R and L to produce a combined left sound signal L- gamma R and a combined right sound signal R- gamma L ( gamma representing a complex number coefficient). These combined sound signals L- gamma R and Lgamma R are fed to the left and right sound transducer means, thereby stereophonic sound is reproduced.

United States Patent Shimada July 1, 1975 STEREOPHONIC SOUND REPRODUCING 3.238.304 moss Yaita 119/1 0 3,478,)? ll/l969 Sorkin l79/l G SYSTEM Primary Examiner-Kathleen H. Clafi'y Assistant Examiner-Thomas DAmico Attorney, Agent, or FirmLewis H. Eslinger; Alvin Sinderbrand [57] ABSTRACT A stereophonic sound reproducing system having left and right audio transducer means arranged relatively close to each other, and a combiner means connected to a stereophonic sound signal source for generating left and right signals R and L and combining the left and right signals R and L to produce a combined left sound signal L-jR and a combined right sound signal RjL (i representing a complex number coeflicient). These combined sound signals L-jR and LjR are fed to the left and right sound transducer means, thereby stereophonic sound is reproduced.

2 Claims, 18 Drawing Figures STEREOPHONIC SOUND REPRODUCING SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a sound reproducing system, and more particularly to a stereophonic sound reproducing system in which an audio transducer arrangement raising stereophonic effect is extremely suitable for miniaturization.

2. Description of the Prior Art With conventional stereophonic sound reproducing systems such as stereo tape recorders, stereo record players, FM stereo receivers, and the like, right and left sounds are respectively reproduced by independent audio transducer systems (each transducer system usually consisting of a single or a plurality of speakers) and the right and left audio transducer systems are disposed, for example, about 1 to 5 meters away from each other, thereby to enhance the stereophonic effect. This inevitably increases the size of the speaker system or a space corresponding thereto. To overcome such a defect, a stereophonic sound reproducing system has been proposed in which right and left speaker systems are disposed close to each other, for example, housed in one box or the speaker systems are miniaturized and disposed close to each other. However, in the case of the stereophonic sound reproducing system being such that right and left sound signals are separately supplied to the right and left speaker systems, differences in intensity and in phase between the right and left sounds from the right and left speaker systems which fall on the listeners right or left ear cannot be recognized so much. Accordingly, excellent stereophonic effects having the same realism as produced in the concert hall cannot be expected with such a conventional stereophonic sound reproducing system.

SUMMARY OF THE INVENTION One object of this invention is to provide a stereophonic sound reproducing system which permits miniaturization of audio transducer systems but produces an excellent stereophonic effect.

Another object of this invention is to provide a stereophonic sound reproducing system which provides an excellent stereophonic effect even when the right and left audio transducers are disposed extremely close to each other.

A further object of this invention is to provide a stereophonic sound reproducing system in which the right and left speakers can be housed in one box.

Still a further object of this invention is to provide a stereophonic sound reproducing system with which sounds produced by the right and left sound speaker systems are combined together to produce the stereophonic effect.

The stereophonic sound reproducing system of the present invention comprises left and right speaker systems disposed close to each other and a combiner circuit for combining together right and left sound signals R and L from a stereophonic signal source to provide sound signals R 'yL and L 'yR. 7' being a complex number coefficient to be applied to right and left speaker systems respectively. The right and left speaker systems are disposed at such a distance that the angle between directions of maximum sound pressures of difference sound components :(L' R) reproduced from the right and left speakers emitting sound components of a wave length corresponding to a practical distance between the human ears may exceed the angle between the ears relative to the center between the right and left speakers.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram showing one example of a stereophonic sound reproducing system of this invention;

FIG. 2 is a circuit connection diagram illustrating one example of a combiner circuit employed therein;

FIGS. 3 to 7 are schematic diagrams for explaining the present invention;

FIG. 8 is a circuit connection diagram showing a modified form of the combiner circuit exemplified in FIG. 2;

FIGS. 9 to 16 are front views illustrating examples of speaker systems of the stereophonic sound reproducing system of this invention; and

FIGS. 17 and 18 are schematic diagrams showing the speaker systems of FIG. IS in their installed condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is illustrated one example of a stereophonic sound reproducing system of this invention, which employs as left and right speaker systems separate left and right speakers 2L and 2R for overall range and in which the left and right speakers 2L and 2R are housed in one speaker box 3. Reference numeral 4 indicates a listener, who stays on the central line between the left and right speakers 2L and 2R, namely on the central axis L Reference numeral 1 designates a stereophonic signal source for generating left and right sound signals L and R, which is such, for example, as a microphone, a tape recorder, a record player, an FM receiver or the like. Reference numerals 5L and SR identify left and right channel amplifier sections respectively supplied with the left and right sound signals L and R from the stereophonic signal source 1. The outputs of the left and right channel amplifier sections 5L and SR are respectively supplied to the left and right speakers 2L and 2R.

In this case, the left and right channel amplifier sections 5L and SR are partly coupled to each other through a coupling means 6 to provide a combiner circuit 7. First and second sound signals S L 3 R and S R 37L (3': being a complex number coefficient) are respectively derived from the left and right channel amplifier sections 5L and SR and are supplied to the left and right speakers 2L and 2R respectively, thus emitting therefrom first and second sounds S, L' y'R and S R1'/L'.

The combiner circuit 7 is of such a circuit construction as depicted, for example. in FIG. 2. In this embodiment the left and right channel amplifiers 5L and SR of the left and right channel amplifier sections 5L and SR are of the emitter grounded type and the emitters of their transistors Q and O are interconnected through a variable resistor 6. The bases of the transistors Q and O are respectively supplied with the left and right sound signals L and R to derive the first and second sound signals S. L-yR) and S R7L)- from the collectors of the transistors 0, and O By altering the resistance value of the variable resistor 6, the

value of the coefficient 7 can be changed in a range of 'y l.

The first and second sound signals S, and S derived from the combiner circuit 7 are respectively amplified by output amplifier circuit such as SEPP amplifier cir cuits and are then supplied to the left and right speak ers.

The aforementioned complex number coefficient is expressed in the form of 37 +j'y,. The real part 'y and the imaginary part 7/, are respectively 0 i l 'y l 1 and 0 Q l y, 1. It is probable that when the coupling means 6 of the combiner circuit 7 includes resistance, electrostatic capacitance and inductance. y 0 and y,

In the present invention the distance d,, (=)\,,/2sim9he tween the left and right speakers emitting sound com' ponents of a wavelength A substantially equal to an auditory sensitive practical distance a between the human ears, that is, the effective distance between the audio sensitive parts of the average listeners ears. namely the distance between speakers 2L and 2R in FIG. I, is se lected such that the angle 26( see FIG. 3) between the directions of the maximum sound pressures (effective or mean values of the sound pressures) of difference sound components :(L R) produced by the left and right speakers 2L and 2R may be greater than an angle 26,, between the human ears relative to the center be tween the left and right speakers 2L and 2R.

Referring now to FIGS. 3 to 7, the operation of the stereophonic sound reproducing system of this invention will hereinbelow be described. In FIG. 3 reference characters SP and SP indicate the central points of sound sources of the left and right audio speakers.

lf-y l 2a, the first and second sounds S, and S-{emitted from the left and right speakers SP, and SP,, are expressed as follows:

The following will discuss the first and second sounds S, and S having a single frequency of a wavelength A.

Reference characters L, to L and R, to R designate circles of sound waves ofthe left and right speakers SP, and SP,, respectively. The circles L, to L are concentric circles about the left speaker SP, which are different in radius from adjacent ones by M2 and the circles. for example, L,, L, and L indicate those positions at which instantaneous sound pressure of the first sound 5, is at maximum in the positive direction and the circles L and L, represent those position where the in stantaneous sound pressure is at maximum in the negative direction. The circles R, to R are concentric cir cles about the right speaker SP which are different in radius from adjacent ones by M2 and the circles. for example, R,, R and R indicate those positions where in stantaneous sound pressure of the second sound 8, is at maximum in the positive direction and the circles R and R, represent those positions at which the instantaneous sound pressure is at maximum in the negative direction.

The sound waves of the first and second sounds S, and S interfere with each other. As above described, the first and second sounds S, and S respectively include sum sound components L R and difference sound components :(I' R) and the sum sound components are of the same phase but the difference sound components are opposite in phase to each other.

Accordingly the sound pressure (the mean or effective value) of each sum sound component L R is at maximum in the direction in which the intersections of the circles L, to L with those R, to R are joined together. In this direction the sound pressure (the mean or effective value) ofthe difference sound components L R) is zero. In the figure black and white circles at the intersections of the circles L, to L and R, to R respectively indicate those positions where the instantaneous value of the sound pressure of the sum sound component L R is at maximum in the positive and negative directions respectively.

Further, the sound pressure (the mean or effective value) of the difference sound component L R is at maximum in the direction in which the intersections of the circles L, to L and R to R are joined together. In this direction the sound pressure (the mean or effective value) of the sum sound component L R is zero. Black and white circles at the intersections of the circles L, to L, with those R to R respectively indicate those positions where the instantaneous value of the sound pressure of the difference sound component L R is at maximum in the positive and negative directions respectively.

The sound pressure (the mean or effective value) of the difference sound component RL =-(LR') is at maximum in the direction in which the intersections of the circles L to L and R, to R, are joined together. In this direction the sound pressure (the mean or effective value) of the sum sound component L+R is zero. Black and white circles at the intersections of the circles L to L and R, to R, respectively indicate those positions where the instantaneous value of the difference sound component LR (R L) is at maximum in the positive and negative directions respectively. Reference characters J 1, and J,, respectively designate directivity characteristic curves of the sum sound component L R and the difference sound components L-R and RL.

The perpendicular bisector ofa straight line connecting the left and right speakers SP, and SP will be referred to as the central axis L,, and the point of the bisector on the straight line connecting the left and right speakers SP, and SP,, is indicated by 0. The direction of the maximum sound pressure of the sum sound component L'+R agrees with the central axis L,, and the directions of the maximum sound pressures of the difference sound components LR' and RL' pass through the point 0 and are symmetrical with respect to the central axis L,, at an angle 9 thereto.

The angle 0 of the maximum sound pressure directions of the difference sound components L'R and RL relative to the central axis L,, is dependent upon the distance d between the left and right speakers SP, and SP and the wavelength A of the sound waves emitted from the speakers SP and SP,,.

Namely. the angle 0 of the direction of the maximum sound pressure of the difference sound component. for example. LR relative to the central axis L,, is such that the phase difference between the difference sound component LR from the left speaker SP, and that R*L (L'-R) from the right speaker SP may be namely M2, as described with FIG. 3 and de picted in FIGv 6. Accordingly. the relationship sin=kl2d hold among the angle 6, the distance d and the wavelength A,

By the way, it is said that a man can readily recognize the direction of the source of a sound which is in a frequency range from about 300Hz to 3KHz only. This is owing to the auditory sensitive practical distance a be tween the human ears.

That is, when we hear a sound whose wavelength is far greater than the auditory sensitive practical distance a between our both ears, even if we turn our heads, the phase difference between sounds falling on our both cars is too small to be recognized by us. Accordingly, it is difficult for us to recognize the direction of the source of a sound whose frequency is lower than approximately 300Hz. Further, when we hear a sound whose wavelength is far smaller than the auditory sensitive practical distance a, even if we turns our heads, the phase difference between sounds falling on our both ears exceeds the wavelength of the sound and is too great to be recognized by us. Accordingly, it is difficult for us to recognize the direction of the source of a sound whose frequency is in excess of about SKHz. However, even in the case of a sound having a fre' quency exceeding 3KHz, we can perceive a little the direction of the sound source by turning our heads several times but the case of a sound having a frequency lower than 300Hz, it is difficult for us to perceive the direction of its sound source even by turning our heads many times.

Turning back to FIG. 3, the following description will be made. The distance d between the left and right speakers SP and SP is selected such that when a listener M stays on the central axis L at a position P where the instantaneous value of the sound pressure of the sum sound L'+R' is at maximum in the positive direction, the angle 20,, between the listeners ears rela' tive to the center between the left and right speakers SP, and SP may be smaller than 26. In this case, let it be assumed that the wavelength of the soundemitted from each of the speakers SP, and SP is M, which is nearly equal to the auditory sensitive practical distance a between the listeners ears.

Thus, the left ear E of the listener M lies between the maximum sound pressure directions of the difference sound component L' -R' and the sum sound component L'+R', while his right ear E lies between the maximum sound pressure directions of the sum sound component L'+R' and the difference sound component R'L'. In such a case, when the listener M turns his head to the left and right, instantaneous sound pressures W and W of sounds (having the aforementioned wavelength A and a single frequency) falling on his ears E, and E vary as shown in FIGS. 4 and 5 respectively, in which the abscisa x represents the distance in a direction away from the point 0 relative to a broken line circle passing the listeners position P and centering about the point 0. That is, the variations in the instantaneous sound pressures W and W with the distance x are of the same phase with respect to the sum sound component L'+R' and are opposite in phase with respect to the difference sound components LR' and R' -L'i It will be understood from the foregoing that the stereophonic effect having the same realism as produced in the concert hall is greatly raised by selecting the distance d between the left and right speakers SP, and SP such that the listener's left ear E lies between the maximum sound pressure directions of the difference sound component LR' and the sum sound component L'+R and the right ear E lies between the maximum sound pressure directions of the sum sound component L'+R' and the difference sound component R'L'.

Accordingly, if the distance d,, (=A /2sin6) between the left and right speakers for emitting sound components having the wavelength A substantially corresponding to the auditory sensitive practical distance a between the listener's cars is selected such that the angle 26 between the maximum sound pressure directions of the difference sound components i(L'-R') produced by the left and right speakers may exceed the angle 26,, between the listener's cars relative to the center between the left and right speakers, the listeners ears may lie between the maximum sound pressure directions of the difference sound components L-R' and R'L' which are the majority of the sound component in the frequency range of about 300Hz to 3KHz that the direction of its sound source can be easily recognized by the human being. This ensures to provide for enhanced stereophonic effect for the sound component of the frequency ranging from approximately 300Hz to 3KHZ the direction of whose sound source can be readily perceived by the human being and which greatly affects the stereophonic effect. According to my experiments, the stereophonic effect was greatly enhanced with the angle 9 exceeding about 10.

In the event of employing as the transducer system a compound speaker system consisting of a tweeter, a squawker and a woofer, two squawkers are provided for the left and right channels respectively and the distance therebetween is selected to satisfy the aforementioned condition d k /2sin0. Further, two tweeters are also provided for the left and right channels respectively and the distance therebetween is selected to be d,, k /2sin0 (A,, being the wavelength of a sound from the tweeters at the center or lower frequency of its range). With such an arrangement, the maximum sound pressure directions of the difference sound components L'R' and R'-L' of the sound produced by the left and right channel tweeters are close to those of the difference sound components L'-R' and RL' of the sound of the wavelength A produced by the left and right channel squawkers, thereby to provide for further enhanced stereophonic effect. This is due to the fact that the direction of the source of the sound having a frequency higher than about 3KHz can also be recognized to some extent as previously described. The woofer may be two in number for the left and right channels respectively, (in which case the distance d therebetween is selected to be greater than d for example, d, M/2sin6, A, being the wavelength of a sound from the woofers at the center or lower frequency of its range) but it is also possible to use one woofer, to supply it with a sum signal of the left and right lowfrequency components and to place to woofer at a desired position, for example, above, below, forwardly or rearwarclly of the listener.

It seems that a man recognizes the coming direction of a sound by perceiving the phase difference between the sound waves at his both cars. This will be explained in connection with FIG. 7. Assume that sound waves of a wavelength A fall on the left and right ears E, and E at an angle I to the central axis L If the phase difference between the sounds at the left and right cars is taken as B, the following equation (1) holds as will be seen from the figure.

. asin=k 2" (1) From this equation the angle 4 is given as follows.

9 sin 1 (2) That is, based upon the phase difference B between the sounds perceived by the left and right ears E and E the man recognizes that the sound is coming from in the direction at the angle to the central axis L For convenience of explanation, let it be assumed that only the left channel sound signal is supplied to the left and right speakers SP, and SP As to a sound of an angular frequency w (the wavelength corresponding thereto is A), the first and second sound signals S, and S respectively bear the following relation to time t.

S L cosmt S, -yI cosmt (4) where 0 1 1 or I y S 0 and L is a constant. First and second sounds (sound pressures) Sf and S produced by the left and right speakers SP and SP, based on the sound signals S and S supplied thereto are as follows:

S, L 'coswt S -y'L coswt (4') where L, and 7' respectively correspond to L and 7 and 0 S *y' l or -l y S 0. Thus, sound fields (sound pressures) S', and 8' near the left and right ears E and E are respectively given by the following equations.

where L, is a factor of the distance x (=0) from the center 0 between the left and right speakers SP and SP to the ears. L, being decreased with an increase in .r and L L' r, is a delay time in propagation of the sound between the central point 0 and the listener's position P and is in proportion to the distance .r and n is a composite coefficient of the difference sound component and is a factor of the distance .r and the distance d between the speakers. n becoming smaller with an increase in and greater with a decrease in d and (l n, l. The first terms of the equations (5 l and (6) are 2 the equations (5) and (6) are respectively as follows:

will: N'Ib The phase difference between S' and 8' corresponds to [3 in the equations l) and (2). B and A are respectively expressed as follows:

Accordingly. from the equations l) and (9) the angle 40 D is given by the following equation.

l tzm (Q F l- :sin i-?-- (Ill The angle 1 may be referred to as equivalent azimuthal angle. Thus. the following facts will be understood from the equation l l That is. the agnle b depends upon the wavelength A of the sound and increases with an increase in the wavelength A. Further. the angle 1 de pends upon the distance d between the left and right speakers. As d decreases, 1 decreases from 1 to 0, and accordingly D becomes smaller.

Let it be assumed that i 1 when 7' O. This implies that the left channel sound L of the wavelength A is emitted from the left speaker only and that the right channel sound R of the wavelength A is produced from the right speaker only.

With an increase in y within a range oft) y l the angle 1 gradually exceeds 1 Namely, the stereophonic sound field becomes expanded. According to my experiments. it has been found preferred that y. namely 1/ is greater than about 0.5.

Further. the angle 1 gradually becomes smaller than i with a decrease in 7' within a range of -l 'y' 0. This implies that the stereophonic sound field becomes narrower.

When the following equation l2) holds.

the direction of a sound cannot be recognized as the phase difference (time lag) between sounds falling on the both ears.

In addition, the following fact will be seen from the equation l). Assume that the distance d between the left and right speakers and the distance 2: between the point 0 and the listener are constant. If A, at the time of 'y'=0 is taken as A,, A A, at the time ofO l and A A, at the time of-l 0 because 1;, is smaller than I. That is, the increase and decrease in A imply an increase and decrease in the sound pressure level (amplitude, mean value and effective value.

FIG. 8 shows a modified form of the combiner circuit 7 exemplified in FIGS. 1 and 2. In the present example, the combiner circuit includes a highpass filter and is adapted not to combine the low-frequency component ofa sound the direction of which the human being cannot recognize.

Reference numerals L and SR respectively indicate, for example, symmetrical emitter-grounded amplifier of the left and right channel amplifier sections 5L and SR of FIG. 1 and reference characters Q and Q amplifying transistors. Reference characters r, and r designate emitter resistors of the transistors Q and O The emitter of the transistor Q, is grounded through a capacitor C, and a resistor r while the emitter of the transistor 0,, is grounded through a capacitor C and the resistor r The resistors r,, r and r and the capacitors C, and C provide a symmetrical high-pass filter HPF. In this case, the cutoff frequency of the high-pass filter I-IPF is selected, for example, about 300Hz. As a result of this, the complex number coefficient for lowfrequency components L, and R, (lower than 300Hz in this example) of the left and right channel sound signals L and R supplied to the left and right transistors Q, and O is nearly equal to zero and the low-frequency components L, and R, are not combined together by the combiner circuit 7 but are derived as L, and R, from the collectors of the transistors Q, and O The middleand high-frequency components L,, and R (higher than 3OOH2 in this example) of the left and right sound signals L and R are combined together by the combiner circuit 7 to provide sound signals (L R and (R, a, at the collectors of the transistors 0 and OR- The first and second sound signals S, and 5-, derived from the combiner circuit 7 are respectively amplified by output amplifier circuits such as SEPP amplifier circuits and then supplied to the left and right speaker systems respectively.

When the frequency of the signal components combined together by the combiner circuit 7 is nearly equal to the cutoff frequency of the high-pass filter I-IPF, the coefficient 1 becomes such that 1, 0 and y, O for the signal components but for the signal components of a frequency higher than the cutoff frequency the coefficient y becomes such that 'y, z 0, y, y and t) y 1) can be altered by changing the resistance value of the resistor r Referring now to FIGS. 9 to 18, several examples of the construction of the speaker system of the stereophonic sound reproducing system of this invention will hercinbelow be described. FIGS. 9 to 16 are schematic from views ofthe speaker box 3, in which elements corresponding to those in FIG. 1 are identified by the same reference numerals.

In FIG. 9 there is shown a two-speaker system in which the left and right speakers 2L and 2R for overall range are attached to the front of a speaker box 3.

FIG. 10 shows a three-speaker system in which left and right speakers 2L, and 2R for middleand highfrequency components are attached to the front of the speaker box 3 and a common woofer 2W is located at the bottom of the box 3. In this case, it is possible to house the woofer 2W in a separate speaker box and to place it at any desired position, for example, above, below, forwardly or rearwardly of the listener.

FIG. 11 illustrates a four-speaker system comprising left and right speakers 2L and 2R for middleand high-frequency components and left and right woofers ZL and 2R which are all attached to the front of the speaker box 3.

In FIG. 12 there is depicted a five-speaker system which has left and right tweeters 2L, and 2R, and left and right squawkers 2L and ZR attached to the front of the speaker box 3 and one woofer 2W placed on the bottom of the box 3.

FIG. 13 illustrates a six-speaker system which comprises left and right tweeters 2L and ZR and left and right squawkers 21. and 2R attached to the front of the speaker box 3 and left and right woofers 2L and ZR attached to both sides of the box 3.

FIG. 14 shows an eight-speaker system provided with four tweeters 2L 2L 2R and ZR left and right squawkers ZL and ZR and left and right woofers 2L and 2R which are all attached to the front of the speaker box 3.

In FIGv 15 there is depicted a five-speaker system comprising left and right tweeters 2L and ZR left and right squawkers ZL and 2R and one woofer 2W which are all located on the front of the speaker box 3.

For example, L1,, 5 cm, (3.4 X IOU/M, 7KHZ, d 23 cm, (3.4 X l0)/k,,=1.5KHz,d,= 100 cm and (3.4 X IOU/A, 34OH2 in the examples of FIGS. 9 to 15.

In the examples of FIGS. 9 to 15 the speaker box 3 may be mechanically reinforced by interposing a reinforcement plate between adjacent spaakers on the front panel of the speaker box 3.

In the case where the left and right speaker systems are compound ones, it is possible to construct the combiner circuit 7 such that in the first and second sound signals S, L 9R and S R yL are different from each other according to the speakers for different sound ranges. Further, the first and second sound signals S, and 8 derived from the left and right channel amplifier sections 5L and SR shown in FIG. I are supplied to filters and then to the respective compound speakers, if necessary, through power amplifiers.

Turning now to FIG. 16, the present invention will hereinbelow be described as being applied to a quasi-4 channel stereophonic sound reproducing system. The left and right speaker systems, in the present example left and right full-range speakers 2L and 2R are attached to the front of the speaker box 3, and, at the same time, left and right auxiliary speaker systems, in this example left and right fulkrange speakers 28, and 2B,, are attached to both sides of the speaker box 3. The distance d,, between the left and right speakers 2L and 2R is selected as previously described and the first and second sound signals S, l. {/R and S R 3 L are respectively applied to the speakers 2L and 2R and, further, first and second suxiliary sound signals S, L RR and S R IiL are respectively supplied to the left and right suxiliary speakers 28, and 2B The complex number coefficient k is generally independent of the aforementioned complex number coefficient Further, the coefficient '5 is usually expressed by It k, +jk,-and SI k l l andOSl k,- I. In this case, it is preferred to select the coefficient If to be equal to 1.

FIG. 17 shows the above-described quasi-4 channel stereophonic sound reproducing system in one condition of installation. This is a fragmentary top plan view of the speaker box 3. The speaker box 3 is positioned at a corner formed by walls W and W perpendicular to each other. First and second sounds S, L y'R' and S R' y'L' from the left and right speakers 2L and 2R are directed to the front of the box 3 and first and second auxiliary sounds from the left and right auxiliary speakers 28, and 2B are directed toward the front of the box 3 after reflected by the walls W and W respectively. The listener stays in front of the speaker box 3.

FIG. 18 illustrates another condition ofinstallation of the quasi-4 channel stereophonic sound reproducing system. in which the speaker box 3 is placed substantially at the center between substantially parallel walls W and W In the quasi-4 channel stereophonic sound reproducing system of FIG. 16 first and second auxiliary sounds S' L k'R' and S' R kL' from the left and right auxiliary speakers 28 and 2B,, are delayed behind the first and second sounds S L y'R' and S R 'y'L' from the left and right speakers 2L and 2R in space. That is, the presence of the first and second auxiliary sounds 8', and which are difference sounds raises the stereophonic effect and the illusion of expansion of the sound reproduced and provides the echo effect.

The present invention described in the foregoing provides a stereophonic sound reproducing system which is simple in construction but extremely high in stereophonic effect.

The foregoing examples are intended as being illustrative and not as limiting the invention specifically thereto and the combiner circuit and the speaker systerm may be modified variously. Further. the kind of speakers need not be limited to a specific one.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

I claim as my invention:

1. A stereophonic sound reproducing system comprising first and second audio transducer means disposed in a common housing and fixed a distance d )t /Z sin 6 apart from each other, where X represents the wavelength of a component of the sound produced by said transducers, which wavelength substantially corresponds to the effective distance between the audio sensitive parts of both ears of listeners, and 6 represents one-half of the angle between maximum sound pressure directions of difference sound components of right and left sounds produced by said first and second audio transducer means and which have said wavelength A,,, a stereophonic sound signal source for generating right and left sound signals R and L. a combiner means for combining said right and left signals R and L to produce a combined right sound signal R- L and a combined left sound signal L- 'yR. 1 being a complex number coefficient, a first circuit means for feeding said combined right sound signal to said first audio transducer means and a second circuit means for feeding said combined left sound signal to said second audio transducer means.

2. A stereophonic sound reproducing system as claimed in claim 1 wherein said combiner means includes a high pass filter for combining together only relatively high frequency components of said right and left signals to which listeners can have sense of direc- 

1. A STEREOPHONIC SOUND REPRODUCING SYSTEM COMPRISING FIRST AND SECOND AUDIO TRANSDUCER MEANS DISPOSED IN A COMMON HOUSING AND FIXED A DISTANCE DA = AO/2 SIN 0 APART FROM EACH OTHER, WHERE AO REPRESENTS THE WAVELENGTH OF A COMPONENT OF THE SOUND PRODUCED BY SAID TRANSDUCERS, WHICH WAVELENGTH SUBSTANTIALLY CORRESPONDS TO THE EFFECTIVE DISTANCE BETWEEN THE AUDIO SENSITIVE PARTS OF BOTH EARS OF LISTENERS, AND 0 REPRESENTS ONE-HALF OF THE ANGLE BETWEEN MAXIMUM SOUND PRESSURE DIRECTIONS OF DIFFERENCE SOUND COMPONENTS OF RIGHT AND LEFT SOUNDS PRODUCED BY SAID FIRST AND SECOND AUDIO TRANSDUCER MEANS AND WHICH HAVE SAID WAVELENGTH AO, A STEREOPHONIC SOUND SIGNAL SOURCE FOR GENERATING RIGHT AND LEFT SOUND SIGNALS R AND L, A COMBINER MEANS FOR COMBINING SAID RIGHT AND LEFT SIGNALS R AND L TO PRODUCE A COMBINED RIGHT SOUND
 2. A stereophonic sound reproducing system as claimed in claim 1 wherein said combiner means includes a high pass filter for combining together only relatively high frequency components of said right and left signals to which listeners can have sense of direction. 